This invention relates to an array of light emitting diodes (LEDs), and, more particularly, to a light emitting diode array structure that has been thermally stabilized by passing currents through heater strips adjacent to the light emitting diodes.
The ability to fabricate closely spaced, or high density, independently addressable light emitting diode sources is important for many applications such as optical disk technology, laser printing and scanning, optical interconnection and fiber optic communications.
Individually, semiconductor light emitting diodes are low power output devices. Arrays of semiconductor light emitting diodes can be used to increase the power output and to simplify optical system design. To provide and maintain good optical alignment of the light emitting elements of the array with one another and to minimize the assembly involved, arrays have been fabricated so that the light emitting elements are in a single semiconductor substrate.
One problem with such arrays is maintaining the electrical and optical isolation between the individual light emitting elements. Another problem is increasing the density of the light emitting elements in the substrate by closely spacing the elements together while still maintaining the isolation, avoiding heat dissipation problems, and providing precise alignment of the elements and the resulting emitted light.
Light emitting diodes are typically arranged in an array with each individual surface emitter light emitting diode producing an individual exposed pixel on a photoreceptor. The individual light emitting diodes in the array are turned on or not turned on in response to a signal corresponding to a digital image.
Typical light emitting diode pixel times for high speed printers are on the order of 10 to 100 nanoseconds. When the light emitting diode is turned on, adiabatic heating of the light emitting diode light emitting areas occurs due to the inefficiency of the conversion of electrical energy into emitted light. The heat dissipates over time periods on the order of 10 to 100 microseconds. This difference in time constants will cause the temperature of the typical light emitting diode light emitting areas to vary with the pattern of the data being written.
Because of the variety of images to be formed with the array of light emitting diode pixels, certain of the light emitting diodes will be operated significantly more than the average while certain other of the light emitting diodes will be operated seldom, if at all, in the short term. This quasi-random mode of operation or duty cycle will vary the degrees of heating for the individual light emitting diodes in the array causing nonuniform temperature induced expansion of sections of the light emitting diode array. These expansions of sections of the light emitting diode array will result in varying the positioning of the light emitted from the array thus varying the pixel positioning on the photoreceptor, losing placement of the pixels, degradation of the image quality, and causing registration errors, especially in the next generation of color printers.
Typical prior art light emitting diode structures may use a heat sink to remove heat from the light emitting diode structure during light emission. The heat sink temperature is maintained at a constant level by using a Peltier or thermoelectric cooler. Because of the thermal resistance between the light emitting diode light emitting areas and the heat sink, this technique is not capable of maintaining the light emitting diode cavity at a constant transient temperature. The heat sink helps maintain an average temperature within the light emitting diode light emitting area. The light emitting diode pixel times for high speed printers occur too fast and over too short periods of time for the heat sink or Peltier or thermo-electric cooler to respond to, thus resulting in temperature fluctuations from pulse to pulse within the light emitting diode light emitting area.
It is an object of this invention to provide a novel means to stabilize the temperature of a light emitting diode array and thus to stabilize the pixel positioning from that light emitting diode array.
It is another object of this invention to provide a means to stabilize the temperature of a light emitting diode in an array at the hotter, higher temperature at which the light emitting diode is emitting light, even when the light emitting diode is not emitting light.
It is another object of this invention to provide a means to stabilize the temperature of a light emitting diode at a constant transient temperature and to stabilize the temperature of a light emitting diode from pulse to pulse.